They were Hollywood’s B films: taut thrillers peppered with character actors that screened before the more pedigreed feature and flourished in the ’40s and ’50s. Fans of the genre can immerse themselves in the world of mean streets, hard-boiled gangsters, gun molls, and two-timing dames when the Harvard Film Archive unspools noir and only noir with a seven-film marathon that runs from Aug. 31 at 7 p.m. to Sept. 1 at 7 a.m.

David Pendleton, who programed “Noir All Night” with HFA director Haden Guest, says the 12-hour marathon will showcase important directors such as Jules Dassin and Abe Polonsky, who were influenced by German Expressionism, Italian neo-realism, and the post-war shift from escapism to grittier subject matter and a more ambiguous morality.